Okay, so the title is a little misleading. It’s not an art, really. It’s simply expanding beyond the “This was so good! I loved it!” line as the entire review and telling the reader why you loved a book. It’s not hard. Just be honest. Otherwise, you’re gushing without depth, and the reader of the review will simply roll their eyes, click “unhelpful” and move on to the negative reviews – which for some reason never fail to go on and on in great detail.
For the love of readers (and authors) everywhere, if you like a book, tell the world about it the right way. Don’t just tell them you like the book. Tell them what it was about it that made you want to write the review in the first place.